There is considerable interest in using optical fibers as the sensing element in devices such as hydrophones and microphones. The possibility of detecting weak magnetic fields by using magnetostrictive perturbations has been discussed in "Detection of Magnetic Fields Through Magnetostrictive Perturbations of Optical Fibers", Optics Letters, Vol. 5, No. 3, March 1980. The subject matter thereof is covered in U.S. patent application Ser. No. 223,635, filed Jan. 9, 1981. The basic principle of operation of such a sensor involves the measurement of longitudinal strain induced in an optical fiber by a dimension-changing magnetostrictive element. When the magnetostrictive material is in the presence of a magnetic field it is caused to undergo dimensional changes which stretch the optical fiber which lengthens its optical path to cause a detectable phase shift in light passing therethrough when compared to light passing through an optical fiber a reference arm which is not affected by the magnetic field.
Detection apparatus constructed according to this scheme is subject to thermal and acoustic disturbances in the vicinity of the sensing arm and this interferes with low frequency magnetic field detection. It is to this shortcoming that the present invention is directed.